<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>字体粗细</title>
    <style>
      div:nth-child(1) {
        font-weight: bold;
      }
    </style>
  </head>
  <body>
    <div>文字1</div>
    <div>文字2</div>
    <div>文字3</div>
  </body>
</html>
